1. Field
The present disclosure relates to a cathode and an electrochemical device. In particular, the present invention relates to a cathode for an electrochemical device, using oxygen as a cathode active material, and an electrochemical device including the cathode.
2. Description of the Related Art
Recently, electrochemical devices, such as a rechargeable secondary battery (for example, a lithium air battery), which use oxygen as a cathode active material have drawn attention. In such electrochemical devices, during discharging, oxygen is supplied from the outside (air or an external oxygen supplying device), and during charging and discharging, the oxygen is used for an oxidation-reduction reaction at an electrode. When a gas, such as oxygen, is used as a cathode active material, a method of connecting a device to a gas bomb (see, for example, JP 2010-528412), or a method of absorbing oxygen from the air by using an air intake in the device (open system) is typically used.
Among these methods, in consideration of decreasing weight or saving space, the method of connecting a device to a bomb is used only with regard to a device for large-capacity power generation and power storage based on a stationary type system. However, this connecting method is generally not suitable for small devices. When air is supplied to a device in which an air intake is installed, impurities, such as water, may penetrate to the device from the air. When an oxidation-reduction reaction of oxygen is used as a cathode reaction, impurities present in a cathode (air electrode) may reduce performance of a catalyst, and during power storage, may deteriorate cyclic performance.
Although these problems may be solved by using a method of filling an electrochemical device with a gas used as an active material (see, for example, JP 2001-273935), filling volume or filling pressure need to be taken into consideration. In addition, a method of controlling air intake by a system (see, for example, 2008-010230) has been disclosed. However, this method may lead to a high energy price due to an increase in the system cost.
In some methods, a partition wall, such as a polymer film, may be installed between a cathode and an air intake wherein oxygen diffuses into the polymer film to prevent the ingress of impurities into an electrochemical device or evaporation of a solvent (see, for example, JP 2007-080793 and JP 2006-134636). However, even when, as in JP 2007-080793 and JP 2006-134636, high oxygen-transmissible materials (for example, a polymer material, such as silicon rubber, disclosed in JP 2006-134636) are used, materials other than oxygen may also be transmitted, and ingress of impurities, such as water or other gas (carbon dioxide in the air), may not be prevented.
Evaporation of a solvent may be prevented by using an ionic liquid with no (or very small) vapor pressure, or ingress of water may be prevented by using water-repellent ionic liquid (see, for example, JP 2011-014478) has been reviewed. However, as in JP 2011-014478, since an ionic liquid is a salt, a small amount of water may permeate into an electrochemical device when it is exposed to the outside air, thereby leading to a decrease in electrochemical characteristics of the electrochemical device.
In addition, a method of disposing cobalt-porphyrin-benzylimidazole complex between a cathode reaction field and an air intake to selectively absorb oxygen (see, for example, JP 2004-319292) has been reported. However, when the method disclosed in JP 2004-319292 is used, the supplying of oxygen to an electrochemical device using oxygen as a cathode active material may be insufficient, because only one oxygen molecule per a unit constitutional molecule size of the complex is chemically bound. Thus, there remains a need for an electrochemical device with improved electrochemical characteristics and efficiency which would continuously supply oxygen as a cathode active material while preventing or suppressing ingress of impurities.